As part of a long-term project aimed at understanding chromium and metallocene catalysed polyolefins, we are seeking to establish a fundamental understanding of structure-property-processing relationships through the control of molecular variables. Through careful selection of polyethylene materials with different molecular weight distributions, level of short chain branching and catalyst type we have been exploring some of these objectives. All our polyethylenes are centred on medium density materials but they differ with respect to molecular variables. Crystallisation kinetics of these materials were studied under isothermal conditions and evaluation of their behaviour has been extrapolated from classical Avrami analysis. By studying the crystallisation kinetics in parallel with melting behaviour, we have been able to correlate thermal transition changes with underlying lamellar characteristics obtained from electron microscopy. This approach allows the distribution of lamellae, thermal stability and molecular constitution to be mapped in real space. In melt-crystallised lamellae, a three tier morphological profile was identified in both chromium and metallocene catalysed polyethylenes. Banded spherulitic structures present in one metallocene narrow molecular sample was absent in the chromium catalysed material. Banding and domaining were observed to dominate the overall crystalline morphology at the longest length scales, dominant lamellar structures at intermediate length scales and subsidiary in-filling lamellar structures at the smallest scale. These different divisions within lamellar texture were highlighted using a combination of electron microscopic techniques following permanganic etching. (C) 2000 Elsevier Science Ltd. All rights reserved.